Can you give us some more information to work with please?
Tank size - volume and dimensions
What fish are in the tank - species and numbers
What are the readings for pH, ammonia, nitrite and nitrate?
What is the GH of your water? If you don't know, look on your water provider's website.
How often do you do a water change, and how much do you change each time?
What type of food do you feed (eg flake, pellets, live food etc)
What chemicals do you add to the tank - presumably water conditioner/dechlorinator but anything else?
The more info you can give, the better it will help to work out what the problem is.
Fasten him for a day and give him boiled deshelled pea, you can give him epsom salt bath by putting him in the quarantine tank or plastic container or whatever you could find, add in water from the main tank then add epsom salt in (epsom salt; 1 table spoon per 1 gallon of water) put him in there for about 20 minutes and then put him back in the main tank. (pea and epsom salt work as a laxative)